What is the difference between Combined column vs Calculated Column. Is there any disadvantage of using Combined column in Tableau? In my scenario I m combining two columns

Col1 and col2 using + sign calculated column str(col1) + str(col2) but in this case if col1 is 0 final data is showing 0. But with combined function its shows 0,col2. but i can not use combined column in a if expression. is there any other way we can correctly merge columns and use in if expression.